This invention relates to jet engine low-pressure turbine assemblies, and more particularly to an improved system for containing ejected turbine blades and other failure events.
An area of recent concern in aircraft engine applications is the containment of low-pressure turbine (“LPT”) blades. Over the past several years incidents have occurred where LPT blades have been liberated from the respective disks and have escaped uncontained from the engine, piercing through the airframe cowling. These ejection events have the potential for hazarding the aircraft.
Normally in LPTs, the shrouds mounted in the casing are relatively thin and are not relied on to provide blade containment. As disclosed in U.S. Pat. Nos. 6,120,242 (Bonnoitt et al.) (“'242 patent”) and 6,468,026 (Bonnoitt et al.) (“'026 patent”), which are incorporated by reference, conventional LPT shroud backsheets were made of thin sheet metal of essentially uniform thickness and served little if any blade containment function. The forward and aft ends of the backsheet are referred to as “rails.” The rails are mounted to corresponding mounting hooks on the casing. Typical conventional backsheet thicknesses range from about 0.015-0.025 inches. The shroud also includes a honeycomb rub strip fixedly joined or bonded directly to the radially inner surface of the backsheet.
In developing blade containment systems for segmented LPT shrouds, considerations include dissipating energy from the blade during an ejection event and reducing the chance of blade ejection or (1) elimination of potential crack growth in the backsheet and (2) improving shear strength of the backsheet at the mounting location. Potential ways to improve containment in existing designs where field experience has indicated that the case is not thick enough to provide adequate containment include increasing case thickness or using thicker shrouds to supplement containment capability of the casing.
Increasing the casing thickness is expensive, as entire casings must be replaced if a current engine is to be retrofitted with a casing of enhanced thickness. Thickening the shrouds raises several issues that must be addressed to ensure a workable design. To improve blade containment, some prior art designs brazed a piece of sheet metal onto the backsheet for reinforcement. This extra piece of sheet metal is commonly referred to as a “doubler.” However, the '242 and '026 patents disclose drawbacks to the “doubler” design because the brazing interface is relatively brittle and subject to cracking, which decreases rather than improves the overall strength of the shroud in a containment event. To overcome these drawbacks of the “doubler” design, the '242 and '026 patents disclose increasing the thickness of the middle portion or “blade containment shield” of the backsheet, but leaving at least one of the backsheet ends, sometimes referred to as rails, at a conventional thickness to permit them to be retrofitted into existing LPT mounting hooks. However, the design contemplated in the '242 and '026 patents also has drawbacks. One such drawback with this design is that the interface between the thicker middle section and thinner rails is a potential weak link in the backsheet, where failure is likely to occur in the event of a turbine blade ejection event. Therefore, there is a need for a backsheet design having the blade containment benefits of a thicker blade containment shield, while avoiding the drawbacks of the thinner rails at the ends of the backsheet and still maintaining the possibility of retrofitting the improved backsheet into an existing LPT shroud.